Electric machinery includes a rotor and a stator. The stator has a stator core, in which motor coils are wound on teeth sections with insulators.
For example, in a vehicle, motors are used for actuating a clutch, an accelerator, etc. A rotary motion of the motor is converted into a linear motion, according to a load, so as to transmit the linear motion.
For example, a clutch mechanism, in which an electric motor is driven, according to an amount of pressing a clutch pedal, so as to actuate an assist member to connect or disconnect clutch plates, a brake mechanism, in which an electric motor is driven, according to an amount of pressing a brake pedal, so as to actuate an assist member to apply liquid pressure of brake oil to a master cylinder, etc. have been developed.
In the above described clutch mechanism or brake mechanism, a three-phase electric motor is often stopped and maintained in a load-holding state where the pedal is pressed and the motor coils are energized. In this state, electric current passing through the coil of one phase will be excessively increased. To solve this problem, one temperature sensor, e.g., thermistor, is provided to the electric motor to measure temperature increase of the electric motor, and temperatures of the phase coils are estimated by calculation. In case that temperature of one of the phase coils is higher than a predetermined temperature, a rotational position of the motor is moved to decrease a current value (absolute value) of the phase, through which the maximum current passes, from a peak value. For example, in case that a great current passes through the W-phase in a state of stopping the motor, the motor is rotated until the current value of the current passing through the W-phase reaches that of the current passing through the V-phase, which is smaller than the peak value of the W-phase, and a current value of the U-phase reaches zero, so that temperature increase of the W-phase coil can be restrained (see Japanese Laid-open Patent Publication No. 2009-220807).
In the above described conventional motor-driven mechanism, if the motor is stopped so as to hold a load, there is a high possibility that temperatures of the phase coils are different. Therefore, a span of life of the mechanism must be shortened due to heat deterioration. Since the temperatures of the coils are detected on the basis of thermal conductive properties of the coils rather than the temperature sensor provided to a motor housing, detection errors will easily occur due to unstable heat conductive properties. Further, the temperatures of the phase coils are estimated on the basis of the temperature detected by the temperature sensor and calculation, so a complex control must be required to perform the calculation process and a parts cost of the temperature sensor must be increased.